1. Field of the Invention
The present invention relates to a voltage-controlled oscillator used for a radio communication device, a radio communication apparatus using the voltage-controlled oscillator and a voltage-controlled oscillation method.
2. Related Art of the Invention
A voltage-controlled oscillator is widely used as a means of generating a local oscillation signal of a radio communication device.
FIG. 12 shows an example of a prior art configuration of such a voltage-controlled oscillator.
In FIG. 12, reference numerals 1a and 1b denote oscillation transistors, 2a and 2b denote inductors, and 3a and 3b denote variable capacitance elements. Reference numeral 4 denotes a power supply terminal, 5 denotes a frequency control terminal, and 6 denotes a current source. A bias circuit and other elements are omitted in FIG. 12.
In FIG. 12, the inductors 2a and 2b and variable capacitance elements 3a and 3b constitute a parallel resonant circuit. As a capacitance value of the variable capacitance element changes due to a difference in voltages at both ends thereof, the capacitance values of the variable capacitance elements 3a and 3b change due to a control voltage applied to the frequency control terminal 5, and consequently a resonance frequency of the parallel resonant circuit changes.
As an oscillation frequency of the voltage-controlled oscillator oscillates in the neighborhood of the resonance frequency of the resonant circuit, it is possible, by adjusting the control voltage, to control the oscillation frequency of the voltage-controlled oscillator to be a desired frequency. The oscillation transistors 1a and 1b are intended to generate negative resistance and cancel losses due to a parasitic resistance component of the resonant circuit so as to satisfy an oscillation requirement.
A relationship between the control voltage and the oscillation frequency of the voltage-controlled oscillator is virtually determined by a characteristic of the variable capacitance element. It is desirable that the variable capacitance element changes the capacitance slowly in a wide range of the control voltage. It is because, in the case of a PLL (phase lock loop) by using the voltage-controlled oscillator, a transient response characteristic and a noise band characteristic of a PLL circuit depend on frequency sensitivity with respect to the control voltage. Therefore, if the frequency sensitivity is different according to the frequency, the characteristic of the PLL circuit itself changes according to the frequency. In an area where the frequency sensitivity with respect to the control voltage is high, there is a problem that a phase noise characteristic is degraded because the frequency changes due to slight noise caused on a frequency control terminal.
In reality, however, it is difficult to utilize the variable capacitance element of high linearity. The reason for this is that when the voltage-controlled oscillator is formed on a semiconductor substrate, costs increase because of a special process needed to form the variable capacitance element. FIG. 13(a) shows the variable capacitance element utilizing a gate capacitance widely used in a CMOS process, and FIG. 13(b) shows the variation of the gate capacitance in the case of applying the reference voltage to the gate of a MOS transistor and applying the control voltage to a drain-source side. Thus, in the case of the variable capacitance element utilizing the gate capacitance of the MOS transistor generally used, the capacitance value abruptly changes in the neighborhood of a threshold voltage (Vth in the drawing). Thus, the oscillation frequency also abruptly changes in the neighborhood of a threshold. Consequently, there arises a problem that the transient response characteristic and noise band characteristic of the PLL circuit using this VCO significantly change depending on the frequency.
The circuit described below has already been proposed in order to solve such problems.
FIG. 14 is a circuit showing a technique of improving the linearity of the variable capacitance element (refer to Japanese Patent Laid-Open No. 2001-352218 for instance). In FIG. 14, the same portions as those previously described are given the same symbols and a description thereof will be omitted.
Reference numerals 10a, 10b, 11a, 11b, 12a and 12b denote the variable capacitance elements, and 13 denotes a level shift circuit. A control signal inputted from the frequency control terminal 5 is inputted to the level shift circuit 13, and voltages having shifted by Vd such as Vt, Vt−Vd and Vt−2Vd are outputted from three output terminals outputted from the above described level shift circuit. In this case, the characteristics of the variable capacitance elements (10a to 12b) against the control voltage Vt are the characteristics shifted by Vd as shown in FIG. 15. As the capacitance of the resonant circuit is a total of these six capacities, a total capacitance thereof is the characteristic indicated by a dashed line in FIG. 15 (A in FIG. 15) so that the change in the capacitance against the control voltage can be moderate.
In the above method, the level shift circuit 13 is constituted by using the transistors such as FET as shown in FIG. 22. This is because the level shift circuit 13 requires high input impedance in order to hold a DC voltage inputted from the frequency control terminal 5.
According to the above method, it is possible to improve the phase noise characteristic of the control signal, but it is not possible to curb the variation or the noise in the power supply voltage. To be more specific, a difference voltage between the power supply voltage and the control signal is applied to both ends of the variable capacitance elements 10a, 10b, 11a, 11b, 12a and 12b, and so they are influenced by the variation or the noise in the power supply voltage even if the noise on the control signal side is curbed. Therefore, the voltage-controlled oscillator changes its oscillation frequency due to the influence of minute variation or the noise in the power supply voltage.
As a countermeasure, there has been proposed a configuration wherein one voltage applied to each variable capacitance element is not the power supply voltage, and each variable capacitance element is interrupted from the power supply voltage with a blocking capacitor so as to supply the reference voltage different from the power supply voltage (refer to “Prospects of CMOS Technology for High-Speed Optical Communication Circuits” by Behzad Razavi, IEEE Journal of Solid-State Circuit, vol. 37, No. 9 September 2002, pp. 1135–1144 for instance).